They fitted against the neighbouring blocks in almost a hair-line.

Its base was slightly curved, and fitted as closely as top and sides upon the huge stones on which it rested.

And then we noted that these stones had been hollowed to follow the line of the grey stone's foot.

There was a semicircular depression running from one side of the slab to the other.

It was as though the grey rock stood in the centre of a shallow cup--revealing half, covering half.
Something about this hollow attracted me.